Why are only two volumes of the Twenty-Four Filial Piety stamps issued?

On September 30, 2014, China Post will issue a set of four special stamps on "Chinese Filial Piety (I)" with a face value of 5.40 yuan. The stamps are 30mm x 50mm. Plate I has 16 stamps per plate with a full-page size of 150 mm x 224 mm, and plate II has 2 sets per plate with a size of 160 mm x 224 mm. The stamps are designed by Lu Yanguang, Cao Guowei and Meng Fudan, and the stamps are printed by Henan Post and Telegraph Printing Plant using engraving and offset overprinting.

Postage stamps (Postagestamp), postage vouchers for posting mail, are generally issued by sovereign states. The square inch space of the stamps, often reflecting a country or region's history, science and technology, economy, culture, customs, natural features and other characteristics, which makes the stamps in addition to the postal value of the collection value. Stamps are also an important source of wealth for some countries or regions. The hobby of collecting stamps is called philately. The world's earliest stamp was the black penny invented by Sir Rowland Hill of the United Kingdom, and China's earliest stamp was the Great Dragon stamp of the Qing Dynasty. To commemorate the 40th anniversary of China's reform and opening up, a large-scale stamp exhibition "China - Great Changes" was held at the University of Leuven, Belgium, from December 11 to 13, 2018, with 188 carefully selected stamps to let the audience relive the historic changes brought by reform and opening up to China.